ABSTRACT We present an all-sky sample of 1.4 million active galactic nuclei (AGNs) meeting a two-color infrared photometric selection criteria for AGNs as applied to sources from the Wide-field ...Infrared Survey Explorer final catalog release (AllWISE). We assess the spatial distribution and optical properties of our sample and find that the results are consistent with expectations for AGNs. These sources have a mean density of 38 AGNs per square degree on the sky, and their apparent magnitude distribution peaks at g 20, extending to objects as faint as g 26. We test the AGN selection criteria against a large sample of optically identified stars and determine the "leakage" (that is, the probability that a star detected in an optical survey will be misidentified as a quasi-stellar object (QSO) in our sample) rate to be ≤4.0 × 10−5. We conclude that our sample contains almost no optically identified stars (≤0.041%), making this sample highly promising for future celestial reference frame work as it significantly increases the number of all-sky, compact extragalactic objects. We further compare our sample to catalogs of known AGNs/QSOs and find a completeness value of 84% (that is, the probability of correctly identifying a known AGN/QSO is at least 84%) for AGNs brighter than a limiting magnitude of R 19. Our sample includes approximately 1.1 million previously uncataloged AGNs.
ABSTRACT We describe the development and application of a Global Astrometric Solution (GAS) to the problem of Pan-STARRS1 (PS1) astrometry. Current PS1 astrometry is based on differential astrometric ...measurements using 2MASS reference stars, and thus PS1 astrometry inherits the errors of the 2MASS catalog. The GAS, based on a single, least-squares adjustment to approximately 750 k "grid stars" using over 3000 extragalactic objects as reference objects, avoids this catalog-to-catalog propagation of errors to a great extent. The GAS uses a relatively small number of quasi-stellar objects (QSOs, or distant active galactic nuclei) with very accurate (<1 mas) radio positions, referenced to the ICRF2. These QSOs provide a hard constraint in the global least-squares adjustment. Solving such a system provides absolute astrometry for all of the stars simultaneously. The concept is much cleaner than conventional astrometry but is not easy to perform for large catalogs. In this paper, we describe our method and its application to Pan-STARRS1 data. We show that large-scale systematic errors are easily corrected but our solution residuals for position (∼60 mas) are still larger than expected based on simulations (∼10 mas). We provide a likely explanation for the reason the small-scale residual errors are not corrected in our solution as would be expected.
We examine the development of mesoscale structure during 3‐D magnetic reconnection which initiates from random perturbations. Reconnection develops as multiple x‐line segments with characteristic ...scale lengths of ∼1–4 Re in the cross tail direction. For relatively wide initial current sheets (several c/ωpi), these finite length x‐lines remain spatially isolated and drive reconnection which is strongly reminiscent of bursty bulk flows (BBFs) in the magnetotail. In narrower initial current layers the x‐line segments merge together to a state in which large scale magnetic energy release takes place. Thus, the degree of compression of the magnetotail may ultimately determine if energy is released in local bursts or globally as a substorm.
There is a growing evidence base demonstrating that atmospheric nitrogen deposition presents a threat to biodiversity and ecosystem function in acid grasslands in Western Europe. Here, we report the ...findings of a workshop held for European policy makers to assess the perceived importance of reactive nitrogen deposition for grassland conservation, identify areas for policy development in Europe and assess the potential for managing and mitigating the impacts of nitrogen deposition. The importance of nitrogen as a pollutant is already recognized in European legislation, but there is little emphasis in policy on the evaluation of changes in biodiversity due to nitrogen. We assess the potential value of using typical species, as defined in the European Union Habitats Directive, for determining the impact of nitrogen deposition on acid grasslands. Although some species could potentially be used as indicators of nitrogen deposition, many of the typical species do not respond strongly to nitrogen deposition and are unlikely to be useful for identifying impact on an individual site. We also discuss potential mitigation measures and novel ways in which emissions from agriculture could be reduced.
UrHip PROPER MOTION CATALOG Frouard, J.; Dorland, B. N.; Makarov, V. V. ...
The Astronomical journal,
11/2015, Letnik:
150, Številka:
5
Journal Article
Recenzirano
Odprti dostop
ABSTRACT Proper motions are computed and collected in a catalog using the Hipparcos positions (epoch 1991.25) and URAT1 positions (epoch 2012.3-2014.6). The goal is to obtain a significant ...improvement on the proper motion accuracy of single stars in the northern hemisphere, and to identify new astrometric binaries perturbed by orbital motion. For binaries and multiple systems, the longer baseline of Tycho2 (∼100 years) makes it more reliable despite its larger formal uncertainties. The resulting proper motions obtained for 67,340 stars have a consequent gain in accuracy by a factor of ∼3 compared to Hipparcos. Comparison between UrHip and Hipparcos shows that they are reasonably close, but also reveals stars with large discrepant proper motions, a fraction of which are potential binary candidates.
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